3rd driver at a great team or 1st driver at mid to back of the pack team?

There are rumors of Vettel being offered a test drive at Ferrari, which at his age pretty much guarantees he will eventually be a full time driver while he's still got a lot of racing left in him.

Now, that might not come for another few years and a racer doesn't like to watch on Sundays. But then again a driver also wants a chance to be champion and that's just not going to happen in his current car. Would you rather drive on Sundays and wait for a chance at a better car that you get to race...say BMW or Renault...or would you jump at a Ferrari test seat knowing one day you'll have your chance to take the season?